Diffusion tensor imaging (DTI) is a form of magnetic resonance imaging (MRI) sensitive to changes in brain white matter. In a recent publication, our group demonstrated the ability of DTI to identify areas of white matter abnormality in postdeployment Veterans in whom the only reported exposure to conditions that might injure the brain was to primary blast forces. Department of Defense (DoD) surveillance data reported 287,861 total diagnosed traumatic brain injuries (TBI) from 2000 through November, 2013, 82.5% of which were mild in severity. It is estimated that 75% of mild TBI sustained during deployment are blast-related. It is likely that many more individuals are exposed to blast during deployment but do not display symptoms consistent with mild TBI (altered level of consciousness, amnesia). Adding to the complexity of the situation, a growing body of evidence demonstrates the potential of repetitive subconcussive events to injure the brain. Most deployment- related events involve a combination of primary blast and other forces, often described as ?blast plus? or blast-related TBI. Preliminary evidence suggests that the initial effects and symptoms of blast-related mild TBI may differ from other injury mechanisms. Differences in injury mechanism(s) and/or injury evolution make it essential to determine the effects in the human brain of exposure to primary blast. We will use multiple advanced neuroimaging techniques to better understand the effect of exposure to primary blast forces on brain structure and function, cognitive function, and symptom presentation. This study will allow us to further investigate the nature of white matter abnormalities observed in Veterans exposed to primary blast forces, providing a better understanding of the possible changes in the brain caused by this exposure. Additionally, we will be able to investigate how these changes may affect brain function by examining alterations in brain networks. We will also examine how exposure to primary blast forces may alter the individual?s ability to focus, concentrate, learn, and perform complex tasks. Finally, we will examine symptoms present following exposure to primary blast forces including anxiety, posttraumatic stress, depression, irritability, poor sleep, and overall quality of life. This study will provide a comprehensive investigation of how exposure to primary blast may affect an individual. Results of this study may have application for both active duty service members as well as Veterans. This study will provide a better understanding of how different levels of primary blast exposure affect brain structure and function. This will improve early identification of individuals at risk for developing white matter abnormalities and allow preventative measures to be taken. Better understanding of the symptoms of exposure to primary blast has the potential to improve evaluation and diagnosis of postdeployment Veterans, thus facilitating provision of appropriate treatments and interventions. Finally, results of this study may provide insight into the chronic effects of mild TBI. Based on our previous experience, all participants will be postdeployment Veterans at least one year out of service. Therefore, changes we see during this study will not be due to the immediate effects of primary blast exposure, and will represent alterations that are not likely to resolve on their own.